A fuel cell is an electrochemical device which reacts hydrogen, as a fuel source, and oxygen, which is usually derived from ambient air, to produce electricity, water and heat. The basic process is highly efficient in fuel cells fueled by pure hydrogen, and it is substantially pollution free. Further, since fuel cells can be assembled into various arrangements, power systems have been developed to produce a wide range of electrical power outputs. As a result of these attributes, fuel cell power systems hold a great deal of promise as an environmentally friendly and valuable source of electricity for a great number of applications.
One of a number of known fuel cell technologies is the proton exchange membrane (PEM) fuel cell. The fundamental electrochemical process under which PEM fuel cells operate is well understood in the art. A typical single PEM fuel cell produces a useful voltage of about 0.45 to about 0.70 volts DC, although most fuel cells are operated at about 0.60 volts DC in order to extract the greatest efficiency from same. To achieve a useful voltage, typically a number of individual PEM fuel cells are electrically combined or coupled in series. In one common configuration, a number of individual fuel cells are electrically coupled in series to form a fuel cell stack. In a fuel cell stack configuration, the anode of one fuel cell is electrically coupled to the cathode of another fuel cell in order to connect the two fuel cells in series. Any number of fuel cells can be similarly stacked together to achieve the desired output voltage and current. An example of a fuel cell which achieves the benefits, noted above, of a stack-like arrangement can be found in our U.S. patent application Ser. No. 11/800,994, and which was filed on May 8, 2007, and which is entitled, “Proton Exchange Fuel Cell Stack and Fuel Cell Stack Module.” The teachings of this pending application are incorporated by reference herein. In another possible fuel cell arrangement, fuel cell stacks are provided wherein the individual fuel cells are separated by an electrically conductive bipolar separator plate. Further, the individual fuel cells are placed between two end plates, and a substantial compressive force is applied to the individual fuel cells positioned between the end plates in order to effectively seal the structure to prevent leakage of the gas and to achieve an operably effective ohmic electrical connection between the respective fuel cells.
Those skilled in the art have long recognized that fuel cell stacks have limitations inherent in their design. To avoid many of the shortcomings and inherent limitations provided in fuel cell stacks, various proton exchange membrane fuel cell modules have been developed. An example of a proton exchange membrane module that fits this description is found in U.S. patent application Ser. No. 11/284,173 which was filed on Nov. 21, 2005 and which is entitled, “Proton Exchange Membrane Fuel Cell and Method of Forming a Fuel Cell.” The teachings of this pending patent application are also incorporated by reference herein. In the proton exchange membrane fuel cells referenced above, each of these devices employ a proton exchange membrane which is typically fabricated from a material called Nafion®. This material has long been the material of choice for proton exchange membrane fuel cells. Nafion® is a copolymer of two monomeric subunits. Those being tetrafluoroethylene (commonly referred to as Teflon®) and perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid. Nafion® is extremely resistant to chemical attachment, and the sulfonic acid groups cannot be broken from the Nafion® even by very strong chemical reactions. Those skilled in the art have typically considered Nafion® to a be a super-acid catalyst. In Lewis acid-based terms, this means that Nafion® can very readily accept a free electron. Therefore, a proton exchange membrane formed from Nation® can stimulate certain types of chemical reactions that require removal of an electron in order to initiate the reaction.
A proton exchange membrane fabricated from Nation® is viewed by many who are skilled in the art as having no pH value. As should be understood, pH is the symbol for the degree of acidity or alkalinity of a solution. It is expressed as the negative logarithm of the hydrogen ion concentration in gram equivalents per liter of solution. It should be understood that the concept of pH presumes that the material in question can be dissolved in water or placed in a solution. Nation® does not dissolve in water and therefore does not have a pH value. If Nation® is immersed in water, the water becomes somewhat acidic. However, the increasing acidity the water is not because the Nation® is adding hydrogen ions to the water. Rather, water is comprised of water molecules which include both hydrogen ions and hydroxyl groups. In this situation, the Nation® absorbs the hydroxyl groups and leaves excessive hydrogen ions in the water causing the pH to decrease. Consequently, Nation®, which is typically employed in most proton exchange membrane fuel cells, is considered neither acidic nor alkaline. Further, it has long been known that in order to render Nation® ionically conductive, water must be present in both its liquid and gaseous forms.
In view of this characteristic of this type of proton exchange membrane, fuel cell designers have long recognized that a fine balance must be struck between the generation and retention of water within the proton exchange membrane, and excessive water must be subsequently eliminated so as to maintain the PEM fuel cell fully operational. Additionally, the management of the heat energy generated by prior art fuel cells have presented unique challenges for fuel cell designers. For example, most designs of proton exchange membrane fuel cells have a maximum operationally effective temperature which cannot be exceeded. This maximum operational temperature is the point at which the heat generated by the operation of the fuel cell causes excessive evaporation of water from the proton exchange membrane. The excessive evaporation initiates a hydration spiral which causes the eventual shutdown (and even irreparable damage) of the proton exchange membrane. Therefore, the design of a fuel cell which addresses the myriad of problems of adequate hydration and management of heat during operation of the fuel cell has been uniquely challenging and difficult for one skilled in the art because prior art fuel cells are often deployed in environments where the ambient temperatures and humidities often widely vary over 24-hour time periods. Such is the case when fuel cells are used in desert regions.
As should be understood, prior art proton exchange membrane fuel cells, as described above, have relatively low operating temperatures, that is, less than 200° C., in relative comparison to other designs of fuel cells, such as solid oxide fuel cells (SOFC). A SOFC is a fuel cell which generates electricity directly from a chemical reaction. Yet unlike a proton exchange membrane fuel cell, a SOFC is typically composed of solid ceramic materials. The selection of the materials employed in such a prior art SOFC is dictated, to a large degree, by the high operating temperatures (600-900° C.) which are utilized by such devices. In view of the higher operating temperatures which are needed to render the ceramic electrolyte of a SOFC ionically active, SOFC devices do not require the use of an expensive catalyst (platinum), which is the case with PEM fuel cells as discussed above. Still further, SOFC devices do not need water to be present so as to render them ionically active, as is the case with proton exchange membranes used in PEM fuel cells. As a result of these higher operating temperatures, assorted fuels can be employed with a SOFC which could not be normally used with a PEM fuel cell. Therefore, a SOFC can directly utilize fuels such as methane, propane, butane, fermentation gases and gasified biomasses, to name but a few.
In a typical SOFC device, a ceramic-based electrolyte formed of a material such as zirconium oxide is sandwiched or otherwise located between a porous ceramic electrically conductive cathode layer, and a porous ceramic electrically conductive anode layer. These cathode and anode layers are typically ceramic gas diffusion layers which are selected for their structural rigidity and high temperature tolerance. A SOFC electrolyte must be impervious to air (oxygen), and must be electrically insulating so that the electrons resulting from the oxidation reaction which takes place on the anode side are forced to travel through an external circuit before reaching the cathode side of the SOFC. In a typical SOFC device, a metal electrically conductive interconnect electrically couples the respective fuel cells in a serial arrangement. If a ceramic interconnect is employed, the selected ceramic material must be extremely stable because it is exposed to both the oxidizing and reducing sides of the SOFC at high temperatures. In the operation of an SOFC device, it should be understood that water is generated as a byproduct of the operation of the fuel cell. However, in these fuel cell devices, water cannot, nor need not, be retained by the ceramic hydrophilic gas diffusion layers employed with same in view of the high operating temperatures (600-900°). Still further, while some amount of water is necessary to render a proton exchange membrane operational, no water is necessary to render the ceramic electrolyte used on the SOFC device operational. Rather, the high temperature of operation of SOFC devices renders the electrolyte ionically conductive. Further, these high temperatures of operation have dictated the use of heat-tolerant, porous, ceramic materials, which are hydrophilic, for the anodes and cathodes of same. In contrast, PEM fuel cells have employed hydrophobic gas diffusion layers in combination with the electrodes employed with such devices in order to manage, at least in part, the effective hydration of the PEM fuel cell.
As should be gathered from the discussion above, the cost of fabricating such SOFC devices have been significant. Further, to render such devices operational, rather complex and sophisticated balance of plant arrangements, and control systems must be employed to controllably heat the SOFC device up to an operational temperature, and then maintain the device within acceptable temperature ranges so as to maintain the ceramic electrolyte ionically conductive.
Other attempts have been made in the prior art to fabricate fuel cells which operate at lower temperatures, and which further employ ceramic anodes and cathodes in connection with a fuel cell which achieves many of the benefits discussed above. In this regard, the Office's attention is directed to U.S. Pat. Nos. 3,297,487 and 4,076,899, the teachings of which are incorporated by reference herein. With regard to U.S. Pat. No. 3,297,487, a fuel cell is described which utilizes an acidic electrolyte and wherein at least one of the electrodes consists essentially of a metal/silicon combination which includes metal/silicon alloys and metal disilicides. In the arrangement as seen in that patent, the electrodes formed from the metal/silicon combinations have the desirable properties of being conductive yet resistant to corrosion, and which might be occasioned by acidic electrolytes, such as, for example, mineral acids like sulfuric acid, and which might be employed in that same invention. Still further, that invention disclosed that the metal/silicon materials which are useful in the fabrication of the electrodes for the fuel cell contemplated by that invention are made from silicon and one or more of the metals selected from the group consisting of iron, cobalt, molybdenum, chromium, manganese, vanadium, tungsten and nickel. The fuel cell as disclosed in U.S. Pat. No. 3,297,487, in one form, employs a ion permeable membrane which substantially prevents contact between the oxidant and the fuel. In this U.S. Patent, the fuel cell is described as generating water by the electrochemical reaction as previously discussed; however, the water generated by the electrochemical reaction which takes place in the fuel cell must be removed to avoid dilution of the electrolyte, and this is typically conveniently done at a temperature above 100° C. by having the entire cell attached to a condenser which selectively removes the proper amount of water. Therefore, in an arrangement such as seen in U.S. Pat. No. 3,297,487, water is produced as a byproduct of the fuel cell operation. However, the water is not necessary to render the electrolyte ionically conductive as is the case with the use of solid proton exchange membranes such as those supplied under the Nafion® trademark.
In U.S. Pat. No. 4,076,899, an electrochemical gas electrode is described and which includes a gas-permeable, conductive mass having a thin, gas-permeable hydrophobic film bonded to its gas-contacting surface. The conductive mass consists essentially of, in one form of the invention, 8-75% by weight of silicon and 25-92% by weight of one or more metals selected from the group which includes vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt and nickel. Still further, the conductive mass may optionally contain a non-essential component of up to 75%, by weight, of a conductive additive which may include a metal, carbon black or graphite. In a fuel cell which incorporates the teachings of U.S. Pat. No. 4,076,899, an electrolyte may be selected which can vary broadly, provided it is inert to the electrodes, and the rest of the fuel cell constituents, and further does not interfere with the fuel or the chemical reactions which takes place within the fuel cell. Acidic aqueous electrolytes are employed and preferred in this prior art invention. One such acidic electrolyte which is preferred is aqueous sulfuric acid because it has a high electrolytic conductivity. Again, in the arrangement as seen in U.S. Pat. No. 4,076,899, water is generated as a byproduct of fuel cell operation. However, the water must be removed from the fuel cell in order to maintain the fuel cell fully operational inasmuch as the water is not required to render the electrolyte ionically conductive and would have the tendency to dilute same.
While traditional proton exchange membrane fuel cell stacks and modules of various designs have operated with some degree of success, a number of shortcomings continue to detract from their usefulness, and have presented quite unique and difficult engineering challenges for prior art fuel cell designers. First among these shortcomings is the relatively high cost of manufacture for the individual components of a traditional proton exchange membrane fuel cell stack. Chief among these high cost components are the bipolar plates which are employed with same. In order to save costs, many manufacturers of fuel cell stacks have attempted to combine a number of functions into the bipolar plates. A modern bipolar plate is a precisely fabricated component that performs a number of functions including fuel management, cooling electrical conductivity, and gas separation. Still further, another problem attendant with the operation of various fuel cell stack designs includes those associated with the management of the proper hydration, and cooling of the fuel cell stack. As noted earlier, some amount of water is necessary to render a proton exchange membrane ionically conductive. However, too much water tends to “flood out” the cell, thereby impairing or stopping the operation of the fuel cell. Still further, while some heat is necessary to achieve an acceptable electrical output, too much heat can cause excessive dehydration which may cause the fuel cell to enter into a hydration spiral where the proton exchange membrane may become breached or otherwise fail to operate effectively. Still further, in proton exchange membrane fuel cell stacks, a heat gradient is typically established throughout the fuel cell stack. Further, “hot spots” may develop, and these hot spots have the effect of degrading the electrical output of the proton exchange membrane fuel cell stack. As should be understood, a number of sophisticated technologies and designs have been developed to manage these hot spots, but the result has been higher manufacturing costs and greater complexity for the resulting fuel cell stack system.
To manage these myriads of problems, sophisticated balance of plant systems have been developed so as to make fuel cells operational in a wider range of ambient environments. However, this has only increased the cost of the resulting fuel cells. The cost of manufacturing, therefore, has been one of several factors which have kept these promising devices from being widely adopted in various industry segments. More specifically, the cost per watt of generated electrical power has far exceeded the cost of electricity taken from most electrical grids. Therefore, fuel cell developers have focused their efforts on reducing the manufacturing costs of the fuel cell by utilizing inexpensive parts and simplified designs so as to lower the price of their products to make them increasingly attractive. Notwithstanding these efforts, the prior art fuel cells have not been widely embraced except in narrow market segments where the costs of manufacture are usually not as important as having a product that can generate electricity for that particular application.
A proton exchange membrane which avoids the shortcomings attendant with the prior art devices and practices utilized heretofore is the subject matter of the present application.